1. Field of the Invention
The present invention relates to microfibrous direct methanol fuel cells (DMFCS) and fuel cell systems that comprise one or more such microfibrous DMFCs, and methods of making and using same.
2. Description of the Related Art
A direct methanol fuel cell (DMFC) generates electricity via the following two electrochemical reactions:    (A) anodic disassociation of methanol/water mixture: 2CH3OH+2H2O→12H++12e−+2CO2↑    (B) cathodic combination of protons, electrons and oxygen: 12H++12e−+3O2→6H2O
The anode and cathode of the DMFC are separated by a proton-conductive membrane (PCM) that is impermeable to electrons, thereby forcing the electrons to travel from the anode to the cathode through a loading-bearing external circuit to generate electrical power.
Polysulfone-based polymers, perfluorocarboxylic-acid-based polymers, styrene-vinyl-benzene-sulfonic-acid-based polymers, and styrene-butadiene-based polymers are all suitable ion exchange polymers for forming PCMs. Commercially available PCMs include the Nafion® membranes supplied by E. I. Dupont, the Flemion® membranes manufactured by Asahi Glass Company, and the Aciplex® membranes by Asahi Chemical Company. Nafion® as used herein is a trademark of E.I. Dupont de Nemours (Wilmington, Del.) for a sulfonated tetrafluorethylene copolvmer. Flemion® as used herein is a trademark of Asahi Glass Co., Ltd. (Tokyo. Japan) for a perfluorinated ionic polymer. Aciplex® as used herein is a trademark of Asahi Chemical Co., Ltd. (Tokyo. Japan) for perfiucrosulfonic acid.
A common problem associated with the PCMs is methanol crossover, i.e., the methanol fuel diffuses through the PCMs and directly reacts with oxygen at the cathode without generating any electricity. Cell potential as well as the energy efficiency of the DMFC can be significantly reduced by methanol crossover. Further, heat generated by methanol oxidation at the cathode is detrimental to the DMFC structure.
Current solutions to the methanol crossover problem include: (1) uses of dilute methanol solutions containing only 3% to 6% methanol, and (2) uses of thicker PCMs. However, dilution of the methanol fuel deleteriously reduces the energy density of the fuel and is not suitable for portable power applications. Use of thicker PCMs, on the other hand, significantly increases the manufacturing cost of the DMFCs, since the PCM is a high cost component of the DMFC.
Further, PCMs demonstrate significant dimensional changes during operation cycles. Hydrophilicity of the ion-exchange polymers used in the PCMs causes excessive swelling of the PCMs upon hydration, and corresponding shrinking upon dehydration. Repeated swelling and shrinking of the PCM have deleterious effects on the cell structure and long-term performance of the DMFC.
It is therefore an object of the present invention to provide a PCM structure of reduced methanol crossover and reduced dimension changes, which is suitable for long-term use with methanol fuel solutions in direct methanol fuel cell systems.
It is another object of the present invention to provide a DMFC system with enhanced fuel efficiency and mechanical reliability.
It is a further object of the present invention to provide a DMFC system capable of using concentrated methanol fuel solutions, i.e., containing at least 30% methanol by weight, more preferably 50% methanol by weight, and most preferably pure methanol, without need for further dilution of such fuel solutions upon actual usage.
It is still a further object of the present invention to provide a DMFC system with reduced size and weight, which can be manufactured cost-effectively through automated production lines and which are particular suitable for use in various portable electronic devices.
Other objects of the present invention will be more fully apparent from the ensuing disclosure and appended claims.